Yury A. Strelenko

Chemistry of N,N-Bis(silyloxy)enamines, Part 5†

Aliphatic nitro compounds can be considered as good precursors of a wide variety of α-azolyl-substituted oximes. The double silylation of convenient aliphatic nitro compounds and the subsequent N,C-coupling of the resulting N,N-bis(silyloxy)enamines 3 with N-silylated azoles 4 lead to the formation of the silylated α-azolyl-substituted oximes 6, which can be smoothly desilylated to give the target α-azolyl-substituted oximes 5. The mechanism of the key step of this process – N,C-coupling – includes the generation of corresponding conjugated nitrosoalkenes 7 (Schemes 4 and 5). The contribution of the chain mechanism in the overall process is considered as well. The studies of the scope and limitations of this reaction, as well as the optimization of its conditions were accomplished. The configuration of the CN bond in oximes was established by NMR.

Stereoselective Free Radical Cycloaddition-macrocyclization in Facile Synthesis of trans -cyclohexano-fused 12-membered Crown Thioethers

Homolytic cycloaddition of dithiols 1,2 derived from trans- and eis-1,2-cyclohexanediols to alkynes, induced by Pr3B-O2, offers an extremely simple approach to trans- and cis-cyclohexano-fused 12-membered crown thialactones 4a-c-7a-c. The reaction of trans-1 proceeds with pronounced remote 1,6-asymmetric induction to give predominantly (IS *, 6R *, 12S *)-4a–c, while cis-2 reacts nonstereoselectively. Basing on molecular mechanics calculations the stereoselectivity is rationalized as a result of entropy favored pathway of macrocyclization.

Conformational analysis of 2-alkyl-1,4-dithianes and related compounds by NMR spectroscopy and MMX

Abstract According to NMR (1H, 13C) spectroscopy and molecular mechanics calculations 2-alkyl substituted 1,4-dithianes exist preferentially in the conformation with equatorially oriented substituents. The conformational free energy for the methyl group in 2-methyl-1,4-dithiane is estimated to be −1.20±0.14kcal/mol. For 1,4-dithianes with substituents of the CH2X type (X = acetoxy, halogen) the predominance of axial conformers is observed originating from an intramolecular electrostatic attraction.

Remote Asymmetric Induction in Free Radical Cycloaddition Leading to Trans-cyclohexano-fused 12-membered Crown Thioethers

Homolytic cycloaddition of dithiol 1, derived from trans-1,2-cyclohexanediol, to alkynes induced by Pr3BO2 occurs with 1,6-asymmetric induction to afford predominantly (1S∗, 6R∗, 12S∗)-trans-cyclohexano-fused 12-membered crown thialactones 4a-c. No pronounced diasteroselectivity was found in the corresponding reactions of dithiol 2, derived from cis-1,2-cyclohexanediol.

Conformational analysis of 2,3-disubstituted 1,4-dithianes by NMR spectroscopy and MMX calculations

Abstract The conformer populations for trans- and cis-2,3-disubstituted 1,4-dithianes have been determined by 1H and 13C NMR spectroscopy and calculated by molecular mechanics (MMX). The CH2X-substituents demonstrate the strong preference for axial position. The remarkable difference originating from intramolecular steric interactions has been observed between two trans-dialkyl derivatives: the trans-2,3-dimethyl-1,4-dithiane is predominantly equatorial whereas trans-2,3-dipropyl-1,4-dithiane prefers diaxial conformation. The properties of cis-2,3-dipropyl-1,4-dithiane are also unusual: its ring interconversion is unexpectedly slow (ΔG≠ = 11.2 kcal mol−1 at 26OK).

Catalytic codimerization of α,β-with γ,δ-unsaturated ketones : novel stereoselective method of the synthesis of functionalized 8-oxabicyclo[3.2.1]octanes

Functionalized 8-oxabicyclo[3.2.1]octanes (3a-e) were obtained in one stage from vinyl ketones and 5-methylhex-5-en-2-one or 1-phenyl-4-methylpent-4-en-1-one in the presence of the catalytic system [RhCl(C2H4)2]2 - SnCl2.

Synthesis of N,N-bis(silyloxy)enamines with a functionalizeddouble bond

The general method for the double silylation of α- or β-functionalized aliphatic nitro compounds 1 leading to regio- and stereoselective formation of N,N-bis(silyloxy)enamines 3 with a functionalized double bond is elaborated. The configuration of obtained products is determined by NMR spectroscopy.